Previously we have isolated a glycoprotein, termed chondronectin, from chicken serum which was utilized by chondrocytes to bind to collagen. Immunological studies showed that the protein was a cartilage specific protein occurring in the matrix around the chondrocytes. Further studies showed that chondronectin binds both to collagen and to the glycosaminoglycan chains of the chondroitin sulfate proteoglycan of cartilage. The affinity of chondronectin for various glycosaminoglycans has been studied and the binding of chondronectin to dextran has been used for the affinity purification of the protein. Using this method, we have recently isolated chondronectin from human serum. Human chondronectin is very similar in size, chain structure and biological activities. Antibody has been prepared to human chondronectin and used to develop an assay for chondronectin in human serum. It is possible that serum chondronectin levels will parallel the metabolic activity of cartilage.